1. Field of the Invention
The present invention relates generally to maintaining spare storage space in a mirrored storage system for use in the event of storage device failure.
2. Description of the Related Art
Disk array systems such as redundant arrays of independent disks (RAID) are used to reliably store data by adding redundancy to the data spread over plural disk drives operating in concert. The present invention is directed to mirrored RAID systems, also referred to as RAID-1 systems. In RAID-1 systems, which are designed to efficiently execute small writes to the storage medium, two identical copies of data are maintained on a disk array. Specifically, the storage devices in RAID-1 systems are arranged in pairs, with each disk of a pair holding data that is identical to the data held by its mate. In contrast, in RAID-5 systems, data and checksum (parity) information is distributed across all drives, such that if a device fails, its data can be reconstituted from the parity information and remaining information on the other devices.
As understood herein, it is desirable to provide spare space in a RAID array, so that if a device fails, its contents may be reconstituted (in the case of RAID-5) or copied over from its mate (in the case of RAID-1) to the spare space, to preserve redundancy. Because of the different configurations of RAID-1 and RAID-5 systems, however, sparing schemes designed for RAID-5, such as those disclosed in, e.g., U.S. Pat. Nos. 5,258,984, 5,657,439, and 5,485,571, are not applicable to RAID-1 systems. Nonetheless, for better performance and even higher reliability, RAID-1 systems remain preferred for certain applications.
Currently, two methods are provided for sparing in RAID-1 systems. The first is simply to provide an extra (2N+1)th device in the system, so that if a device fails, the content of its mate can be copied onto the spare. Unfortunately, in some implementations, such as arranging RAID devices on a large, field-removable card known as a “blade”, space is at a premium. There may simply not be enough room for the extra spare device.
The second method currently provided for sparing in RAID-1 systems is to relinquish an entire pair of mated devices for use as spares. This, of course, reduces the capacity of the system by an entire pair of devices.
Having recognized the above problems, the solutions below are provided.